Heart beat apps

Have any of you tried those heart beat apps to hear your baby's heart beat in between doctor's appointments? I'm 24 weeks, my first time being pregnant, and I haven't felt a lot (or any) movement and I'm starting to get concerned. It has been 6 weeks since my last doctor's appointment because she has been on vacation and I was really looking forward to my appointment which was supposed to be today. However they canceled so now I have to wait another week and a half to get some peace of mind. So I was wondering if anyone has tried an app to listen to the heart beat and was successful? If so, what would you recommend? Thank you!!